The present invention relates to an electrophotographic color image forming apparatus in which a plurality of charging means, image exposure means and developing means are arranged along the circumferential surface of a drum-shaped image-forming member, and toner images are formed and superimposed while the image-forming member is rotated by one revolution.
Concerning the method for forming a multi-color image, there have been known some methods including apparatus (A) in which photoreceptors, charging units and developing units each in quantity equivalent to the number of colors necessary for the multi-color image are provided, and toner images each being a mono-color formed on each photoreceptor are superimposed on an intermediate transfer member to form a color image, apparatus (B) in which one photoreceptor is caused to make plural turns so that charging, image exposure and developing for each color are repeated for forming a color image, and apparatus (C) in which charging, image exposure and developing for each color are conducted in succession while one photoreceptor makes one turn for forming a color image.
However, the apparatus (A) has a drawback that the dimensions of the apparatus are increased because a plurality of photoreceptors and intermediate transfer meters are required, while the apparatus (B) has a restriction that the size of a formed image is limited to the surface area or less of the photoreceptor although the dimensions of the apparatus can be small because the required number of each of the charging means, image exposure means and photoreceptor is just one.
In the case of the apparatus (C), which makes it possible to form images at high speed, it still has a contradiction that the diameter of a photoreceptor is large and thereby the apparatus is also large due to the following two reasons; one is the necessity that a plurality of charging units, image exposure means and developing units need to be arranged within a circumferential surface of the photoreceptor, and the other is the necessity that the distance between the image exposure means and the developing unit needs to be long for avoiding a possibility that image quality is deteriorated by toner leaking from the developing unit to which an image exposure optical system is located close.
For the purpose of avoiding the drawback of the aforementioned contradiction in the apparatus (C), there has been suggested an apparatus in which the base of an image-forming member is formed from a transparent material, a plurality of image exposure means are housed in the image-forming member, and a light-sensitive layer formed on the external surface of the base is exposed to light reflected on an image through the base (for example, Japanese Patent Publication Open to Public Inspection No. 307307/1993).
However, in the apparatus suggested above, since the image exposure means are provided inside the image forming member and the great number of charging devices and developing devices are mounted outside the image forming member, the construction inevitably becomes complicated. Further, a mounting and dismounting operation for the developing devices, the image forming member and the image exposure means becomes troublesome and the handling operation for the machinery components may be not easy. Also, it may be difficult to maintain a predetermined precise positional relation among the machinery components. Still further, to rotate the image forming member together with the optical exposure system incorporated in the image forming member and to mount or dismount the image forming member requires a difficult technique.
In particular, the optical exposure system is arranged with high precision among LEDs and between each LED and the image forming member. Accordingly, if deformation or positional deviation takes place on the optical exposure system at the time of mounting or dismounting the image forming member, the registration of the superimposed images or the image forming position may be deviated, resulting in that a good quality image may be not obtained.